Surface reaction rate

I want to model the O2 consumption on tissue surface as Michaelis-Menten equation, which states:

R=Vm*Ci/(K+Ci)

where Vm and K are constant and Ci is O2 concentration at the wall. In UDF (DEFINE_SR_RATE) Ci is defined by the molar concentration, therefore I have to convert it by multiplying the mass fraction of O2 by density and divide it by the molar weight, am I right?. I think the density must be defined at the centre not the face and in surface reaction is defined at the face, so how can I get the density at the cell centre? please help!!.